Biosynthesis of phytoquinones. Incorporation of l-[Me−14C3H]methionine into terpenoid quinones and chromanols in maize shoots

Abstract

Radioactivity from L-[Me14C,3H]methionine is incorporated into phylloquinone, plastoquinone,[gamma]-tocopherol, [alpha]-tocopherol, [alpha]-tocopherolquinone and ubiquinone in maize shoots. Comparative studies with other terpenoids (squalene and [beta]- carotene) and chemical degradation of selected quinones (ubiquinone and plastoquinone)established that all the radioactivity is confined to nuclear methyl substituents. In ubiquinone 76% of the radioactivity is in the methoxyl groups and 24% in the ring C-methyl group. Taking the phytosterols as an internal reference and accepting the atomic ratio of 14C/3H transgerred from L-[Me-14C,3H]-methionine to the supernumerary group at C24 to be 1:2 the ratio of all the quinones and chromanols examined approached 1:3. After allowing for the fact that for plastoquinone, [gamma]-toeopherol, [alpha]-tocopherol and [alpha]-tocopherolquinone one nuclear methyl group is formed from the [beta]-carbon of tyrosine, these results show that one nuclear C-methyl group for phyllopuinone, plastoquinone and [gamma] -toeopherol, 2 nuclear methyl groups for [alpha]-tocopherol and [alpha]-tocopherolquinone and one nuclear methyl and 2 methoxyl groups for ubiquinone are formed by the transfer of intact methyl groups from methionine. From a comparison of the incorporation of 14C radioactivity into these compounds it would appear that the methylation reactions involved in phylloquinone and plastoquinone biosynthesis take place in the chloroplast, whereas those involved with ubiquinone biosynthesis occur elsewhere within the cell.